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More than 17 million voters were eligible to vote in 55 seats where polling was being held on Wednesday

Voting has begun in crucial assembly elections in India's most populous state of Uttar Pradesh.

A high turnout of about 62% was reported on the first day of a seven-phase election that ends on 3 March and has nearly 126 million eligible voters.

Uttar Pradesh is ruled by Chief Minister Mayawati, a low-caste Dalit icon who is battling another strong regional force, the Samajwadi Party.

But the poll is also a major test for the beleaguered ruling Congress party.

If it were a separate country, Uttar Pradesh would be the fifth largest in the world by population, with more than 200 million people.

More than 17 million voters were eligible to vote in 55 seats where polling was held on Wednesday.

Uttar Pradesh Election Facts

Voting dates: Feb 8, 11, 15, 19, 23, 28, March 3

Vote counting: 6 March

Eligible voters: 125.8m

Assembly seats: 403

Cost of election: 3bn rupees ($61m)

Police and paramilitary troops deployed: 500,000

Election officials: 500,000

Polling booths: 135,000

Polling booths opened at 07:00 on Wednesday but officials said turnout was fairly low in the initial hours because of heavy rain in many parts.

But as the weather improved, larger numbers of voters came out, with election officials recording a 62% turnout.

Chief election officer Umesh Sinha congratulated voters for their enthusiasm, saying the figure was the biggest since independence.

The average turnout in the 2007 state election was 46%.

Turnout campaign

Political heavyweights from India's governing Congress party and the main opposition Bharatiya Janata Party (BJP) have been campaigning extensively in the politically vital state.

At the scene

By Mukesh SharmaBBC Hindi, Uttar Pradesh

A sudden early morning downpour spoiled the show. The roads in most parts of Bahraich district are poor. Many have huge potholes and, with the rain, were barely navigable. In some places, water also entered the polling stations.

There was not a lot of enthusiasm among the voters. Many polling booths were totally deserted.

But in the Kaiserganj constituency, I saw a long queue outside a polling station. Here voters complained they had been waiting for two to three hours to cast ballots because the polling officer was taking too long. Some left without voting.

The Election Commission's recent campaigns to motivate voters to come out to booths did have some impact. Ruchi Jaiswal, 27, who has a physical disability, said she had never voted before because she thought no-one had raised issues important for her and other disabled people. But this time she said she had been convinced that every vote counted.

The state, one of India's most backward, sends the largest number of MPs to parliament and has provided the country with eight prime ministers since independence in 1947.

"Nearly half a million police and paramilitary troops have been deployed for the month-long election," Mr Sinha told the BBC.

He said an equal number of polling officials would be manning the state's 135,000 election booths.

Election officials have held several events, such as cricket matches, marathons and rallies, over the past month to try to motivate people to vote.

The elections will decide the fate of Ms Mayawati's Bahujan Samaj Party (BSP). She is hoping to repeat her stunning victory of 2007, when her party won 206 of the 403 assembly seats.

Analysts, however, say the government has been mired in controversies and corruption cases and Mayawati is unlikely to repeat her performance.

Her main challenger in the elections is the regional Samajwadi Party, led by its youthful leader Akhilesh Yadav, the MP son of former chief minister Mulayam Singh Yadav.

The Samajwadi Party mainly represents the interests of a caste grouping called Other Backward Classes and the father-son duo has been campaigning vigorously across the state.

Congress, too, is hoping to boost its presence in the state, with MP Rahul Gandhi leading the party's campaign effort.

Recently, he has also been joined by his mother and party president Sonia Gandhi and sister Priyanka Gandhi.

Some polls have suggested that Congress could increase its vote and may hope to ally with the Samajwadi Party to oppose Ms Mayawati.

The state elections are expected to be a litmus test ahead of national elections, which are due in 2014.

Manipur, Punjab, Uttarakhand and Goa are also voting in this round of state elections.

A hefty turnout was reported in Punjab and Uttarakhand elections.

In Manipur, the first round of polls was marred by violence when suspected rebels attacked a polling station killing four people.